Dehumidifying systems are used in a variety of different applications. For example, air dehumidifying systems are used in applications ranging from dehumidification of offices for maintaining comfortable working areas during summer months to providing dry air for dental tools. Different applications often require different levels of humidity. A humidity level of about 40% to 60% is comfortable in homes or offices, while a humidity level of less than 10% is desirable in certain laboratory situations. Even lower humidity levels are often desirable in communications systems.
Commonly used signal transmission media in communications systems are waveguide, coaxial cable, multi-wire telephone cables, and optical fiber cables. Changing environmental conditions can affect the overall performance of a system using any of these media. For example, when the temperature of air inside a waveguide or other transmission medium falls below its dew point, condensation occurs inside the transmission line. Condensation lowers the efficiency of waveguide and coaxial cable systems partially because the dielectric constant of water is greater than the dielectric constant of air, and partially because the condensation alters the impedance of the waveguide or coaxial cable and may produce signal variation or loss. In multi-wire cables, condensation can lower the insulation resistance and introduce undesirable leakage paths.
To prevent the accumulation of moisture in such systems, the transmission line is normally sealed and pressurized to prevent the ingress of moisture through any small gaps. To prevent condensation within the system, the pressurization is effected with dry air from a dehumidifier or dehydrator. A compressor or pump typically supplies the pressurized air, and the dehumidifying apparatus removes moisture from the pressurized air before it is injected into the system. The low moisture content of the air lowers the dew point so that condensation does not take place except at very low temperatures. Moreover, due to the small amount of moisture present in the injected air, only a small amount of condensate can form even at unusually low temperatures.
One of the known types of dehumidifiers uses a membrane cartridge to remove fluid from a gas that passes through the membrane cartridge. The membrane cartridge contains multiple membranes through which moisture and a portion of the gas being dried permeate the membrane and escape to the atmosphere or a collection system. The membranes are typically in the form of hollow fibers so that a gas may be passed through the interiors of the fibers while moisture removed from the gas is collected from the exteriors of the fibers.
Present dehumidifiers that use membrane cartridges generally use a compressor to supply pressurized air to the membrane cartridge. In a typical dehumidifier of this type, the desired dew point of the dehumidified gas is achieved by maintaining a constant flow rate of the gas through the membrane cartridge while keeping the pressure constant. These systems require relatively large compressors in order to maintain a constant flow rate and pressure within the dehumidifying system at higher elevations, i.e. at relatively low ambient pressures.